CAKE: Preheat oven to 350 degrees F. Grease and flour 9 X 13 X 2-inch pan. Combine flour and sugar in large bowl. Melt butter, add cocoa and Pepsi. Pour over flour and sugar mixture, and stir until well blended. Add buttermilk, beaten eggs, soda, and vanilla. Mix well. Stir in marshmallows. Pour into prepared pan. Bake 40 minutes. Remove cake from oven and frost while still warm. FROSTING: Cream Butter, sugar, and peanut butter. Add milk and stir well. Add nuts. Spread over warm cake. Place frosted cake under broiler about 4-inches from heat source. Broil just a few seconds, or until topping starts to bubble. DO NOT scorch! Let cool at least 30 minutes before serving.